Spinal fusion surgery is currently the state of the art treatment for a variety of lumbar region pathologies and mal-adaptations that have not responded well to rehabilitative therapies. Conditions necessitating the procedure include degenerative disk disease, severe spinal compression, fractures involving neurological damage, spinal stenosis, tumors and structural instabilities caused by conditions such as scoliosis. Fusion typically involves the use of implantable constructs that facilitate calcification between adjacent levels (segments) of the spine. Pedicle screw and rod constructs are currently widely used to secure adjacent levels by means of metal rods passing through screws embedded on the posterior pedicle regions of the spine. The rigid orientation of these rods allows for alignment and distraction of the spine to a certain degree, diverting the load borne by the fused regions and inhibiting further bone degradation and neurological injury at the fused regions.
Fusion implants have evolved tremendously from early Harrington rod constructs, which utilized thick straight rods and hooks to secure the spine. These presented a high risk of post-operative fracture and tended to apply adverse mechanical stress on adjacent spinal segments. Conventional pedicle screws and rods are highly contoured and machined to adapt to different placement configurations and present the lowest possible profile. They utilize minimally invasive drilling and placement techniques that allow for rapid patient recovery and improved prognosis. However, in spite of these advances, studies indicate that about 43% of lumbar fusion patients develop adjacent segment disease (ASD) requiring fusion of the adjacent levels as well. The immobilization of one set of spinal levels is compensated by increased mobility requirements and mechanical stress on the facet joints of adjacent levels, leading to spine degeneration and stenosis. G. Cheh et al., Spine 32, 2253-2257 (2007).
With about 150,000 to 300,000 lumbar fusion procedures being carried out annually, there exists a significant need to secure adjacent spinal levels through revision surgery in the least invasive means possible. J. M. Cottrell et al., HSS Journal the musculoskeletal journal of Hospital for Special Surgery2, 12-18 (2006). Barriers to revision surgery include potential dural tear and root injury related complications as well as the formation of scar tissue following the fusion procedure. The success rate for revision surgery is reported to be 60-80%. K.-S. Cho et al., Journal of Korean Neurosurgical Society 46, 425-430 (2009).
Currently, ASD is surgically addressed by fusing adjacent segments using an additional pedicle screw-rod construct (a highly invasive option), or by implanting inter-spinous spacer devices (ISPs). Posterior insertion of ISPs is conducted by placing the ISP between the spinous processes of the segments requiring stabilization. ISPs have been found to benefit elderly patients who have a high risk of complications, osteoporosis or poor general health. A major disadvantage of ISPs is that they are dependent on the distraction of intact and healthy spinous processes. In many cases, lumbar fusion surgery involves a laminectomy or removal of the inferior spinous processes belonging to the fused segments. Thus, these spinous processes are not available to properly seat an ISP. Insertion of conventional ISP devices also involves the removal or disruption of the posterior ligamentous complex (PLC), which is critical to the stability of the spine.
Some static ISP devices do not firmly attach or adhere to the spinous processes and have been implicated in increased osteolysis at the contact points between the implant and bone tissue. For example, the X-Stop® interspinous spacer by Medtronic Inc. features a cylindrical shaft that distracts adjacent spinous processes. The shaft is held in place by compressive forces exerted by an adjustable clamp. Osteolysis at the point of contact between the implant and bone is a concern due to micro-movements of the implant during its effective lifetime. The Vertiflex Superion® is an interspinous spacer similar to the X-Stop, but one which may be implanted into the lumbar spine under local anesthesia on an out-patient basis. The spacer is delivered through a small midline incision and flaps expand from the columnar body of the spacer to secure it between adjacent spinous processes Once implanted, the spacer acts as a support column to open nerve and cord passageways.
Osteomed's PrimaLok™ SP device is a posterior supplemental fixation device intended to temporarily fix the thoracic, lumbar or sacral spine while awaiting bony fusion to take place. The device is intended for use at one level with bone graft material, and is not intended for stand-alone use. Four polyaxial gripping plates are incorporated into the device to grip opposing spinous processes. The polyaxial nature of the plates allows for variation in patient anatomy, unlike the static configurations used in some of the other devices. Due to its temporary nature, implant loosening or migration are possible complications of the PrimaLok SP. Due to its placement with the spinous processes, fracture of the spinous processes is also a possible complication. See Osteomed®, primaLOK™ Surgical Technique Guide, 2-17 (2012).
Other conventional implants are intended for rigid fixation with spinous processes. For example, the Medtronic Spire™ System is a spinous process plating system that may supplement Medtronic's CD Horizon® rod and screw system. The Spire System, which may be used as a fusion device, consists of two spiked, titanium compression plates. A compression instrument is used to clamp the two plates together, driving the spikes into bone tissue of adjacent spinous processes. A locking screw achieves rigid fixation of the device. For proper placement and anchoring, the Spire System requires two intact spinous processes. The Aspen® Spinous Process Fixation System (by Lanx®) is a two plate compression system classified as an adjunct to fusion and designed as an alternative to pedicle screws. The Aspen system includes a spiked fixation plate designed to be rigidly affixed to the spinous processes with the use of specialized surgical instruments. A hollow central area serves as a bone graft chamber to facilitate fusion, and also contributes to the implant's ability to share weight load of the spine. The Aspen system is intended for single level use in the thoracic or lumbar spine (T1-S1) for the treatment of degenerative disc disease, spondylolisthesis, spinal trauma or spinal tumor. Implant migration and spinous process fracture are possible complications of the Aspen system. Shah, Mitesh V., TLIF with Unilateral Pedicle Screws and Aspen™ Spinous Process Fixation System, Spine Universe Case Study Library, http//www.spineuniverse.com/professional/case-studies.
The Coflex-F® is also a non-pedicle supplemental fixation device, including a u-shaped body for resisting relative motion between adjacent levels, with opposing pairs of plates on both sides. The plate pairs include apertures for two pins, allowing the pairs of plates to be press-fit to opposing spinous processes, with the pins passing through holes that are punched in each of the spinous processes. Safety and efficacy of the device for spinal stabilization without fusion has not been established. The Coflex-F is intended for use with an interbody cage and autogenous bone graft, as an adjunct to fusion at a single level in the lumbar spine (L1-S1). Paradigm Spine, Coflex-F™, http://spinerevolution.com/wp-content/uploads/2011/08/coflex_F_detail1.pdf. As with other of the above-referenced implants, migration and spinous process fracture are possible complications of the Coflex-F device. Regence Medical Policy Manual, Surgery—Interspinous Distraction Devices (Spacers), Policy No. 155 (2006).
U.S. Pat. No. 8,016,860 to Carl et al. seeks to reinforce the spine or correct a spinal deformity with a hood-like element that is positioned over and screwed or bolted onto the spinous process of a first vertebra. A pair of screws connect to the hood at first screw ends and angle outward to connect to pedicle screws that are anchored to opposing pedicles of a third vertebra, at opposing second ends. A cross bar between the two pedicle screws prevents them from creeping laterally. Carl et al. addresses the above-noted problem of spinous process fracture by suggesting that a cavity be created within the spinous process and filled, with a reinforcing material in order to strengthen and support the bone prior to attaching the Carl et al. device.
U.S. Pat. No. 6,793,656 to Matthews also makes use of a cross-member. Matthews discloses a temporary, suprafacial spinal fixation device including pedicle screws with bilateral fixation plates engaged thereover. A pair of linking members is secured between the pedicle screws to laterally connect the plates. The plates are secured to the pedicle screws and the linking members to the plates via top-loaded nuts.
U.S. Pat. No. 6,709,435 to Lin discloses a three-hook device for fixing the spinal column. A first hook is inserted into the spinal canal of a spinal segment, while two length-adjustable hooks engage the spinal projection of the segment. A pair of rods are inserted into apertures on either side of the hook device, so that multiple vertebrae may be fixed by attaching additional three-hook devices to the rods, to engage other vertebrae.
U.S. Patent Application Publication No. 2007/0233082 by Chin describes a pivoting clamp that engages and locks the spinous processes of adjacent vertebrae. Long bolts may be used to anchor the device transversely through a spinous process. U.S. Patent Application Publication No. 2008/0183218 by Mueller is similar to the Chin device, being a clamp with protrusions for engaging and frictionally securing the spinous processes of adjacent vertebrae. However, Mueller does not require boring through a spinous process.